You can use {label} or {html} placeholders to let the renderer know where where the element label or the element html are positionned according to the error message. They will be replaced accordingly with the right value. The error message will replace the {error} place holder. For example: <font color="red">{error}</font>{html} will put the error message in red on top of the element html.
If you want all error messages to be output in the main error block, do not specify {html} nor {label}.
Groups can have special layouts. With this kind of groups, the renderer will need to know where to place the error message. In this case, use error blocks like: <!-- BEGIN form_group_error -->{form_group_error}<!-- END form_group_error --> where you want the error message to appear in the form.